A radio set comprising a conventionally publicly known radio antenna apparatus is shown in FIG. 17 so as to schematically show an antenna and related parts. The radio set of the prior art is constituted by an external antenna 602 such as a whip antenna or a helical antenna, a built-in antenna 603 such as a plane antenna, feeder lines 604 and 605, a transceiver unit 606 including a transceiver, and a microphone 609 connected to the transceiver unit 606, which are provided in a radio set housing 601. The external antenna 602 and the built-in antenna 603 are arranged in proximity to each other so as to be electromagnetically coupled with each other, constitute a receiving space selective diversity antenna. The external antenna 602 is arranged so as to be electrically insulated from the radio set housing 601, while a predetermined point of the built-in antenna 603 is grounded to the radio set housing 601 through a short-circuiting line 603a, and the built-in antenna 603 constitutes an inverted-F antenna.
When a power is supplied to the external antenna 602, a switch 607 is turned on so that the external antenna 602 is connected to the transceiver unit 606 provided in the radio set housing 601 through the feeder line 604. At the same time, the switch 608 is turned off, and the feeder line 605 connected to the built-in antenna 603 is disconnected from the transceiver unit 606.
On the other hand, When the built-in antenna 603 is supplied with power, the switch 608 is turned on so that the built-in antenna 603 is connected to the transceiver unit 606 through the feeder line 605. At the same time, the switch 607 is turned off so that the feeder line 604 connected to the external antenna 602 is disconnected from the transceiver unit 606.
In the radio set comprising the conventional radio antenna apparatus described above, the external antenna 602 and the built-in antenna 603 are designed to have a high gain primarily in a free space, and have a uniform horizontal plane directivity or radiation pattern along the x-y plane with a center of the external antenna 602 and the built-in antenna 603. In other words, as shown in FIG. 17, in the case where the orthogonal coordinates are set so that the z-axis direction is coincident with the axial direction of the external antenna 602 and the x-axis direction is coincident with the direction of the normal to the built-in antenna 603, the horizontal plane directivity pattern of the antenna of the conventional radio set in a free space is shown in FIG. 18, and it has a shape of a circle (as indicated by a thick solid line of FIG. 18) with the center of the z-axis on the x-y plane, as shown in FIG. 18. It is to be noted that the microphone 108 is arranged under the radio set housing 101 on the side nearer to the whip antenna 102 in the x-axis direction.
The conventional radio antenna apparatus described above has the same horizontal plane directivity pattern in the x-y plane and hence a horizontal plane non-directivity pattern. Therefore, in a case where a human head or the like obstacle approaching the microphone 609 exists in proximity to the radio set comprising the conventional radio antenna apparatus described above, the radio wave is interrupted by the obstacle, and this leads to a problem of gain deterioration.
An object of the present invention is to solve the above-mentioned problems and to provide a radio antenna apparatus, in which the horizontal plane directivity pattern of the antenna is changed in a direction not affected by an obstacle, and radio wave interference by the obstacle is reduced so as to improve a radiation efficiency thereof.